Is your Joomla site in the root folder of your site? Check, that you have configured the same url address to your Analytics profile and Joomla.
E.g. If your Joomla is under
http://www.mysite.com/joomla/, and in your profile settings your url is
http://www.mysite.com, remember to put your tracking code to your root directory "http://www.mysite.com/index.htm" also. Google will try to find the tracking code from the url, you have typed in your profile settings.
Also it takes 24-48 hours from Google to enable the tracking. How long has it been, since you added the code?